The US thanked Oman on Thursday for its assistance in ensuring the temporary release of an American-British-Iranian citizen held in Iran.
Oman maintains close ties with Washington and Tehran and has acted as a go-between for the two countries on many issues.
"I extended our warmest thanks to Oman for its help in achieving the furlough of Morad Tahbaz," US Special Envoy for Iran Robert Malley said on Twitter.
"It is high time that he and the three other unjustly detained US citizens return home," Mr Malley said.
Mr Malley's statement comes after he held talks with Oman's Foreign Minister Badr Al Busaidi.
Oman's Foreign Ministry said the two officials spoke about "various issues in common and efforts to resume the nuclear agreement known as the Joint Comprehensive Plan of Action".
Mr Al Busaidi has previously said that he was optimistic that all parties would return to the 2015 Iran nuclear deal and that his country was prepared to facilitate regional discussions with Tehran.
However, efforts to revive the nuclear pact have stalled in recent months owing to differences between Iran and the US.
The 2015 deal gave Iran sanctions relief in exchange for restrictions on its atomic programme to guarantee that it would not develop a nuclear weapon ― something that the country has denied seeking.
But US president Donald Trump abandoned the deal in 2018, calling it too soft on Iran, and re-imposed harsh US sanctions. In response, Tehran began breaching the nuclear limits in the pact.
Who is Morad Tahbaz?
Mr Tahbaz, 66, was released from prison on bail on Wednesday with an electronic tag and has returned to his home in Tehran.
“The Tahbaz family have confirmed Morad has been released from Evin prison on furlough and is at their home in Tehran," said a statement by the British Foreign Ministry.
“Morad is a tri-national and we continue to work closely with the United States to urge the Iranian authorities to permanently release him and allow his departure from Iran," the statement said.
He is an environmental activist and was arrested in 2018 by Iranian authorities.
They accused him of working as a spy while tracking endangered animals in Iran.
The Iranian judiciary sentenced Mr Tahbaz to 10 years in prison for “contacts with the US enemy government” in November 2019.
His arrest was part of a crackdown on environmental activists.
